In the production of metals by the continuous casting method, molten metal is transferred from a ladle to an intermediate tundish that has one or more nozzles for transferring the molten metal in the tundish to the mold from which the continuously cast metal is manufactured. In general, the molten metal contains undesired impurities from sources such as particles of refractory from the lining of a vessel that contains the molten metal, alumina as a byproduct of deoxidation and reoxidation, fragments of slag or other insoluble impurities. Upon solidification of the cast product, these impurities adversely affect product properties such as surface finish, drawability, weldability, formability and strength.
It is, therefore, desirable to remove any remaining unwanted impurities from the molten metal immediately prior to casting. Ceramic foam filters and cellular filters, as they are known in the art, have been shown to be capable of removing unwanted impurities from high temperature molten metal. Aubrey, Ceramic Foam--An Effective Filter for Molten Steel, presented at the Fifth Intl. Iron & Steel Congress, Washington, D.C., Apr. 7-9, 1986.
U.S. Pat. No. 4,940,489 describes a molten metal filtration system whereby a dam is provided on the discharge side of a ceramic filter wall to collect molten metal, such that hot molten metal is present on both sides of the ceramic filter to prevent freezing of molten metal in the filter element during priming of the filter element. In the actual practice of this system, the flow rate of molten metal is too slow and the filter plugs too quickly, such that the usable life of the ceramic filter is short as compared to the tundish life. As a remedy, it has been suggested that passages be placed through the filter wall, allowing molten metal to flow unfiltered to the tundish discharge. The passages are placed in the filter wall without any specific consideration concerning the size of the passage, other than to freely permit the flow of metal even when the filter elements are completely plugged. I have found that this remedy is inefficient because it allows large amounts of molten metal to pass through the wall unfiltered, and as little as 5% of the molten metal passes through the filter and is actually filtered.
U.S. Pat. No. 5,064,175 describes a method whereby depressions are fabricated into the refractory surface that contacts the molten metal in a tundish vessel. The resulting depressions on the refractory surface provide for greater surface area for inclusions in molten metal, such as alumina, to adhere. The present invention, providing for protrusions from a continuous flat or nearly flat refractory surface, operates quite differently. Refractory protrusions into the molten metal stream from the refractory wall operate to disrupt the surface boundary flow to create eddies in the molten metal adjacent to the protrusions, as the molten metal circulates along the refractory surface. The eddies provide an optimum location where fluid flow conditions and surface conditions are favorable for alumina to be deposited. This aspect of the present invention offers enhanced deposition because it is not relying on increased surface area to increase deposition of alumina, but relies upon disruption of fluid flow at the fluid/refractory boundary.
Accordingly, it is an object of the present invention to provide a molten metal purification and filtration apparatus which increases filtration efficiency of molten metal.